Solvent extraction process for plutonium



United. States Patent SGLVENT EXTRAGTION PROCESS FOR PLUTONIUM' Glenn. T. Seahorg, Chicago, Ill., assignor to the United States. of America as. represented by, the. United States Atomic Energy Commission No Drawing. Application October 12, 1945 Serial. N 0., 622,097

The present invention relates to the solventextraction ofheavy'metal compounds, and particularly to the extraction: of a compound of a transuranic element from an aqueous solution thereof by means of an organic solvent.

An object of this invention isto provide amethod for the separation of a transuranic element-from aqueous solutions thereof.

Another object of this: inventionisto provide a method for the separation and purification of a transuranic element contained in aqueous solutions of said elementaud other elements of lower atomic weights.

A further object of the, present invention/is. to provide suitable organic solvents.- and extraction, procedures. for the separation of element94 from.aq ueous. solutions there.- of.

Other objects and advantages of this. invention will be evident from the following description.

The term element. 94 is used inthi's. specification to designate the element of atomic number 94', which is. also referred to herein as plutonium, symbol Pu. It has recently become known that various. isotopes. of element 94 can be prepared. by a number of diiierent. nuclear processes. 238, referred to as 94 can be. prepared by deuteron bombardment of the uranium isotope. 9.2 to produce; 93 followed by beta decay of the 92 to 94 (The isotope 93 is the mass 238 isotope of the..element. of.

atomic number 93., referred to as neptunium, symbol.

Np.) The plutonium isotopes 94 and 94 can. be

prepared by neutron bombardment of the uranium isotope of. mass; 3.8:

Neutronic reactors for theproduction of'element 94 from'natural uranium by a self sustaining chain reaction have recently'been developed. Oneof theisotopes occurring in. natural uranium is 92 which is usually present to the extent of about 0.71 percent by weight. When this isotope is bombarded by" slow neutrons, preferably' of thermal energies; it undergoes fission and releases on. an average of about two neutrons per neutron; ill-addition to fission. fragments of relatively low atomic weights. The isotopes 94 and 94 can also undergo'fission when bombarded by neutrons; and these fission processes also release an average of about two neutrons perneutron absorbed in the" fission reaction: Thus, in'a natural-uranium neutronic reactor, the excessrneu trons released by fission are sufiicient to maintain. production of element 94 through neutron absorption by the predominant uranium isotope 92 The concentration of plutonium so produced is generally small, rarely being above one percent by weight of the uranium and usually being substantially below such concentration.

Element 94 may suitably. be. separated. from masses of uranium which have undergone neutron bombard- Thus, theplutonium isotopeof massv plutonium. more strongly than. doesthe nitrate ion. The:

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ment forperiods'oftime sufficient to produce recoverable amounts of'plutonium, but which may be. insufficient to destroy more than a minute fraction of the total uranium. through fission of 92 and. neutron absorption by 92 It may be desired" to recover the plutonium content of such uranium masses. having plutonium concentrationsbelow 1 part per. thousand. and even below 1 part per.v million parts of'uranium.

Neutron-bombarded uranium, or a neutron-bombarded uranium compound, will 'contain,.in addition to plutonium' and unreacted' uranium, various radioactive fission prod-- nets in a, total concentration ofthe same order of magnitude as the plutonium'concentration. Theoriginal frag: ments produced by'fission are highly unstable radioactive isotopes of short half-lives which. rapidly decay to longer lived isotopes. The fission products which are present? after practicable aging periods will usually range in. atomic numbers from about 32 to about 6'4 and in atomic. masses from about 77 toabout 158. These fission. prod"- ucts compriselargely a light group in which isotopesv of Sr; Y, Zr, Cb, and Ru are. the most dangerouslyradios active; and a heavy group. in which isotopes of Te, I, cs. Ba, La, Ce, and Pr are the most dangerously radioactive.

In the recovery of plutonium from neutron-bombarded. uranium, the plutonium is conveniently processed in aqueous solutions, and preferably in aqueous, inorganic. acid solutions. Thus, in the initial separation of p111.- tonium from thebulk of the unreacted uranium and fission products, in thefurther decontamination of the plutonium by separation from. residual radioactive. fission.- products, and in the final concentration, purification, and recovery of plutonium,. it is desirable to efiect the plutonium separations from aqueous nitric acid or other suit.- able aqueous medium. Various precipitation methods have been suggested for the separation of" plutonium from such aqueous solutions; These methods, however, have. generally entailed the use of large amounts of auxiliary carrier precipitates, and thehandling of very. large, volumes of solutions.

In accordance: with the present invention; plutonium compounds may besseparated from aqueous nitric acid solutions, aqueous; hydrochloric acid solutions; or other suitable aqueous media, by extraction into" organic solvents of the class hereinafter described. This solvent extraction procedure. may be employed for the separation of plutonium from aqueous solutions. in. any. phaserof the" above-mentioned procedure for. the recovery of luto. nium from neutrond'rradiateduranium- Inaddition, the. solvent extraction procedure; offthis. invention may be. used for the separation of. plutonium compounds from aqueous solutions other. than .those; derivedv directlyfrorn neutron-irradiated. uranium. Such solutionsmayrange; from waste liquors containing only traces of plutoniumto relatively concentrated. solutions. of. puresplutonium compounds.

The aqueous solutions fromwhich plutonium, may." be extracted in accordancev with the. present inventionsuit ably contain plutonium. ions..of.at.least -l- 4 valence state,. said ions being substantially free from strong complex? ing agents. Preferably, no ion should be. present inthe aqueous solution which forms. ionic. complexes hydroxyl ion, and the. anions of acids. which ionize in. aqueous solutions to a substantially smaller degree than. nitric acid, are particularly disadvantageous from the-- standpoint of complexing tetravalent plutonium. Thus, hydroxyl, sulfate,. phosphate, fluoride, and oxalate ions tend to. complex tetravalent. plutonium sufficiently to.- decrease its extractability into organic solvents; Ibis therefore. desirable to minimize the: concentrations: Ofi" free ions of this class in the aqueous solutions to be t 3 extracted. Most of the complexing ions can be excluded in the preparation of the aqueous solutions. Alternatively, an interfering ion may itself be complexed by another ion, or its concentration as a free ion may otherwise be minimized by control of the concentration of another ion which cancombine with it. Thus, the fluoride ion can be complexed by zirconyl ion, and the hydroxyl ion can be supressed by hydrogen ion, to reduce their interference with the extraction of tetravalent plutonium. v

The extraction of hexavalent plutonium is relatively free from interference by hydroxyl ion.v Hexavalent plutonium can be extracted from aqueous solutions of any hydrogen ion concentration sufficient to prevent the precipitation of a basic plutonium compound.- An aqueous solution of plutonyl nitrate may thus be extracted without the necessity of free nitric acid in the solution. In the case of tetravalent plutonium, on the other hand, it'is desirable to maintain free acid in the aqueous solution. Solutions of plutonous nitrate for extraction with organic solvents should have a pH not substantially above 2.5, and preferably should have a concentration of free nitric acid of at least 1 N.

In orderto improve the distribution of plutonium betweenthe' aqueous and organic phases, it is generally desirable to incorporate a saltin'g-out agent in the aqueous solution. A salting-out agent, for this purpose, has the same characteristics as a salting-out agent for previously known solvent extraction processes, i.e., high solubility inthe solution to be extracted and low solubility in the extract phase. Likewise, it'will be apparent that the presence .of high concentrations of any substances which tend to dissolve in the solvent phase more readily than theplutonium, is to be avoided. If such a condition is permitted to occur a salting back of any plutonium present in the solvent layer takes place, thus tending-to greatly decrease the efficiency of this process. The preferred salting-out agents for use in the present invention are those having a common ion with respect to the compound being extracted. Thus, if a nitrate of plutonium is being extracted, the salting out agent is preferably an inorganic nitrate. Examples of suitable salting-out agents for this purpose are:

The concentration of the salting-out agent which is desirable in any particular case will depend on the valence o'f'thecation and the concentration of the common anion due to any free acid in the solution. In the case of 1'- -N nitric acid solutions, .it is desirable to employ a concentration-of a univalent nitrate of at least 3 M, and

, and the oxygenated organic solvents are the preferred v 4 acidic aqueous solutions as well as neutral solutions,

extractants for use in the present process.

Although most normally liquid organic compounds containing an atom with excess electrons, such as oxygen, sulphur, or nitrogen, are capable of forming a coordination bond, it will be evident to those skilled in the art that certain molecular structures can interfere with this electron-donating property. Electron-attracting constit- Bis-fl-chloroethyl ether uents such as halogen atoms can off-set the electrondonating property of an atom such as oxygen,, if present in sufiicient number and proper relationship to the donor atom. For this reason it is preferable to employ compounds containing only carbon, hydrogen, and electrondonor atoms. It will also be apparent that certain molecular configurations can give rise to steric hindrance which may interfere 'sufliciently to prevent the formation of coordination bonds. Tertiary carbon atoms adjacent an electron-donor atom and long chains of non-donor atoms linked to a donor atom areespecially undesirablein this respect. The preferred solvents are those in which the donor atom is linked to a hydrogen atom or to nontertiary carbon atoms and in which at least one component linked to the donor atom contains less than four consecutive non-donor atoms.

The following are examples of suitable solvents for use in the present process:

Ethyl ether Heptadecanol Z-ethylbutanol I Methylisobutylcarbinol Methyl ethyl ketone Methyl amyl ketone Methyl isobutyl, ketone Z-phenoxyethanol 2benzyloxyethanol 2-(p-ethylbutoxy)ethanol 1,2-diethoxyethane In employing any of the solvents of the above class in the present process, previously known extraction procedures and apparatus may-be employed. The extraction may be elfected by batch, continuous batch, batch counter-current, or continuous counter-current methods.

increased or decreased with decrease or increase in acid concentration.

-The extraction agents which are suitable for use in my process comprise normally liquid organic solvents which are substantially immiscible with the aqueous solution to be extracted and which contain at least 1 atom capable of donating an electron pair to a coordination bond. Such solvents suitably comprise water-immiscible organic compounds containing an oxygen, sulphur, or nitrogen electron-donor atom. It will be evident, how ever, that most nitrogen-containing organic compounds of this type are basic in nature and will be unsuitable for the extraction of acidic aqueous solutions. Such compounds may be used, if desired, to extract substantially neutral solutions containing small amounts of hexavalent plutonium. Most organic solvents containing oxygen or,

sulphur donor atoms maybe used for theextraction of The most eflicient extraction is obtained in continuous counter-curent operation. The usual types of extraction,

equipment and the usual operating procedures may be employed when effecting the present extraction in this manner.

feed at the bottom of the column, and solvent draw-off at the top of the column. The top section of such a column may serve as a stripping section, and an auxiliary stripping medium may optionally be charged to the top of the column. a

The plutonium may be recovered from the solvent extract phase by any suitable procedure such as evaporation of the solvent, crystallization with an isomorphous .which are insoluble in the organic solvent employed.

The purification effected by this procedure is especially Satisfactory extraction in accordance with this procedure may be obtained by the use of a packed column with aqueous feed at an intermediate point, solvent est m4 advantageous in separating "plutonium from the fission products contained in solutions derived from neutromirradiatednranium. This procedure is also useful in si- "niultan'eous'ly extracting *both plutonium and uranium from solutions of neutron bombarde'd'uranium. Re eateh 'extracti onslwith one 'o'rlmo're solvents may be used to effectpurification and concentration 'of plutonium to an extent sufficient to enable final recovery of a pure compoundofplutouium.

The present invention will be further illustrated with particular reference to certain preferred classes of solvents.

One preferred class of solvents for use in the present invention comprises ethers containing at least one oxygen atom capable 'of donating an electron pair to a coordinationbond. The most desirable compounds of this class are the saturated aliphatic ethers, and it 'has been found preferable to employ a'dialkyl ether containing at least one oxygen atom whichis attached by an ether-linkage "to non-tertiary aliphatic carbon atoms, at least onecomponent linked to said oxygen atom containing less than four consecutive n'on-donor atoms. The following examples illustrate the use'o'f solvents of this class:

Example '1 A solution of PuO in 1 M HNO M Ca(NO was agitated with an equal volume of ethyl ether until extraction equilibrium was obtained. The phases were then separated and the extract was analyzed for plutonicontent on the basis of its alpha radiation and that o'f'the original aqueous solution. The plutonium content of the extract was "found to be941percent of that in the initial charge. H

Example :A :solution "of P110 Kin 1 'HNO -IO M N'H NO was charged into a'n extraction column-and continuously extracted with approximately 5.4 times its volume of ethyll ether. All the conclusion of the extraction,- the extract and r aifinat'e were analyzed for plutoniumfcontent on the basis of the al-pha radiation of the initial charge and of the resulting solutions. The plutonium distribution was found to be as follows:

Example 3 :Uranyl. nitrate hexahyd'rate, which had been "subjected toineutron irradiation from 50 milliampere hours deuteron bombardmentof beryllium and aged for80 days,'wa's melted, acidified with 70 percent "nitric acid to 0.66 HNQ made 0.02 -'-M in K' l'rO with solid potassium dichron'late, and held at 60 C. for a'bo'ut'one hour to effect oxidation of the "plutonium to PuOg' The solution wasthen cooled to roomtemperature and extracted with'approximately 533 times its "volume of ethyl ether, ina single stage abatchex'traction. The extract was found to contain 86 percent of the pluto'nium content ofth'e aqueous "charge, together with a substantially greater percentage not the uranium content -of the charge, but only 8 ;perce'nt of the fission product's.

Example 4 solution of Bu :in .1 1HNO3-103M NH NO was agitated with an equal volume of ethyl ether until the extraction equilibrium was attained. The phases were then separated and the-extract was analyzed for plutonium content on the basis'of its alpha radiation and that of the original aqueous solution. The plutonium content of the extract was found to be 35 (percent of that in the initial charge.

The substitution of normal propyl or isopropyl ethersv for the ethyl ether in the above examples williresult in lessadvantageous distribution coefiicients, but adequate ex- 56 traction is obtainable in flicient counter-current opera tion.

The'sulphur analogues of'the others are also useful in my process, and the following example illustrates the use "of a solvent of this type:

Example 3 A'solution of PuO-f? in l M HNO -lQfM NH NO was agitated with an equal volume of ethyl sulfide until the extraction equilibrium was attained. The phases were then separated and the extract was analyzed for'plutonium content on the basis of its alpha radiation-and that of the original aqueous solution. The plutonium content of the extract was found to be 43 percent of that in the initial charge.

The glycol ethers comprisef-a'preferred class of solvents having especially favorable distribution coefficients for plutonium extraction. The desirable solvents of this class are saturated glycol ethers containing at least one oxygen atom'which is attached byan ether vlinkage to'a non tertiary aliphatic carbon atom, at least one component linked to said oxygen atom containing less than four consecutive non-donor atoms. ,The preferred ethers of this class are the monoglycol monoalkyl ethers, polyglycol monoalkyl ethers, monoglycol dialkyl ethers, and polyglycol dialkyl ethers. The following are examples oftheuse of sol-vents of this class:

Example 6 was charged into an extraction column and continuously extracted with approximately 6 times its volume of 2(3- ethylbutoxy)ethan0l. At the-conclusion of the extraction, the extract and rafiinate were analyzedfor. plutonium content. The plutonium distribution was found to be=as follows:

Percent Extract .8346. Raiiinate 14.4 Loss 2.0

. :Exa'mple 8 A solution of Pu+ in 0.6 M'UO tNOQ -l HNO 10 'M NI-I NO was 'agitated'with an equal volume of 2(i8-ethylbutoxy) ethanol until the extraction equilibrium was attained. The phases were then separated and the distribution of plutonium and uranium betweenthe aquewe and organic phases was determined. The percentage extraction of each of the metals fromthe initial aqueous solution was found to be approximately as follows:

Percent Plutonium 60 Uranium "63 Example 9 A solution of Pu+ in .1 1M 'HNO3-10 M -NH NO, was agitated with an equal volume of 2-benzyloxyethanol. After extraction equilibrium had been attained, the phases were separated and the extract was analyzed for plutonium content. :It was-found that the extraction was approximately 48 percent complete.

Another preferred class of solvents "for use in the present extraction process comprises normally liquid organic solvents containing at leastone c'a'rbonyloxygenatom capable of donating an electron pair to"acoordination-bond. This class includes carbocyclic acids, esters, aldchydes ,7 and ketones. The most desirable solvents of this group comprise aliphatic ketones and especially dialkyl ketones having a carbonyl group attached to at least 1 non-tertiary carbonatom. The following are examples of the utilization of solvents of this class in the present process.

Example 10 A solution of Pu+ in 1 M I-INO -IO M NH NO was-agitated with an equal volume of methyl ethyl ketone until the extraction equilibrium was attained. The phases were then separated and the extract was analyzed for plutonium content on the basis of its alpha radiation and that of the original aqueous solution. The plutonium content of the extract was found to be 78.9 percent of that in the initial charge.

- Example 11v A solution of Pu in 8 M HCl was agitated with an equal volume of methyl isobutyl ketone. After extraction equilibrium had been attained, the extract was analyzed for plutonium content. The extraction was found to be 91 percent complete.

Example 13 Y W i A lanthanum fluoride precipitate containing plutonous fluoride (Pu+ was dissolved inaqueous zirconyl nitrate andthe resulting solution was made 3 M with respect to nitric acid and M with respect to ammonium nitrate. The concentrations of zirconium, lanthanum, and plutonium in this solution were approximately 18.6, 6.2 and 0.0001 g. per liter, respectively.

The above solution was charged continuously to a' packed extraction column at a point approximately 0.4 of the column length below the top of the column. Methyl isobutyl ketone was charged continuously to the bottom of thecolumn at a rate'approximately double the charge rate of the plutonium solution, and an aqueous stripping solution, comprising 3 M HNO 5 M NH NO was continuously charged to the top of the column at a rate approximately one-half the charge rate of the plutonium solution. The total charge to the column, per hour, was approximately 0.9 of the column hold-up.

After reachingequilibrium, the methyl isobutylketone extract, which was continuously withdrawn from the top of the column, and the spent aqueous solution, which was continuously withdrawn from the bottom of the column, were analyzed for plutonium content. The extract was found to contain approximately 99.75 percent of the plutonium in the charge solution, and the spent aqueous solution contained the residual 0.25 percent.

Example 14 e A solution of Pu?" in 1 M HNO M NH NO was agitated with an equal volume of acetophenone until the extraction equilibrium was attained. The phases were then separated and the extract was analyzed for plutoniurn content on the basis of its alpha radiation and that of the original aqueous solution. The plutonium content of the extract was found to be 89 percent of that in the original charge.

Example 15 A solution of PuO in 1 M HNO -10 M NH NO was agitated with an equal volume of menthone until the extraction equilibrium was attained. The phases were then '8 tent. The extraction was found to be approximately 41 percent complete. H x p A further class of preferred solvents for the presentinvention comprises normally liquid organic solvents containing at least one nitro oxygen atom capableof vdonating an electron pair to a coordination bond. Desirable solvents of this class comprise nitrohydrocarbons, and especially nitroalkanes. The following are examples of the use of nitrohydrocarbons in'tlie present process;

Example 16 A solution of Pu+ in 1 M HNO 10 M NH NO was agitated with an equal volume of nitromethane until the extraction equilibrium was attained. The phases were then separated and the extract was analyzed for plutonium content. The extraction was found to be approxi mately 58 percent complete. 7

' A solution of PuO in 1 M HNO3-10 M Mario,

was agitated with an equal volume of nitroethane until the extraction equilibrium was attained. The phases were then separated and the extract was analyzed forj'plu tonium content. The extraction was mately 61 percent'complete.

Example '19 A solution of PuO in saturated aqueous lithium-nitrate was agitated with 1.5 times its volume of l-nitrospropane until the extraction equilibrium was attained. The phases-were then separated, and the extract was found to contain approximately 81 percent of the plutonium content of the original aqueous solution. I

Example 20 A solution of Puo,+= in 1 M HNO -IO M NH NO was agitated with an equal volume of nitrobenzene. The phases were then separated and the extract was analyzed for plutonium content. The extraction was found to be approximately 28 percent complete.

It is to be understood, of course, that the above examples are merely illustrative and do not.limit the scope of my-invention. Other solvents of the class previously described may be substituted for the specific solvents of these examples, and the procedures employed may be' modified in numerous respects within the scope of the. foregoing description. In general it may be said that the use of any equivalents or modifications of procedure which would naturally occur to those skilled in the art is included in the scope ofmy invention. Only such limitations should be imposed on the scope of this invention as are indicated in the appended claims.

This is a division of copending application U.S. Serial No. 591,410, filed May 1, 1945, which .is a continuationin-part of copending application U.S. Serial No. 481,660, filed April 3, 1943, and patented on October 29, 1957, as U.S. Patent No. 2,811,415, and all subject matter therein not inconsistent with the subject matter herein is incorporated by reference. 1

What is claimed is:

1. A process for the separation of plutonium from anaqueous solution containing plutonium in an oxidation found to be approxistate of at least +4 which comprises contacting said soseparatedand the extract was analyzed for plutonium contract and raffinate'phascs.

2. A process for the separation of plutonium from an aqueous inorganic acid solution containing plutonium in an oxidation state of at least +4 and a common ion salting-out agent, which comprises contacting said solution with a solvent selected from the group consisting of nitromethane, nitroethanc, l-nitropropane, nitrobenzene and o-nitroanisole, and separating the resulting extract and raffinate phases.

3. A process for the separation of plutonium from an aqueous nitric acid solution containing tetravalent plutonium ions and a nitrate salting-out agent, which comprises contacting said solution with a solvent selected from the group consisting of nitromethane, nitroethane, l-nitropropane, nitrobenzene and o-nitroanisole, and separating the resulting extract and rafiinate phases.

4. The process of claim 3 in which the nitro compound is nitromethane.

5. The process of claim 3 in which the nitro compound is nitroethane.

6. The process of claim 3 in which the nitro compound is l-nitropropane.

7. The process of claim 3 in which the nitro compound is nitrobenzene.

8. The process of claim 3 in which the nitro compound is o-nitroanisole.

9. A process for the separation of plutonium from an aqueous nitric acid solution containing hexavalent plutonium ions and a nitrate salting-out agent, which comprises contacting said solution with a solvent selected from the group consisting of nitromethane, nitroethane, l-nitropropane, nitrobenzene and o-nitroanisole, and separating the resulting extract and raffinate phases.

10. The process of claim 9 in which the nitro compound is nitromethane.

11. The process of claim 9 in which the nitro compound is nitroethane.

12. The process of claim 9 in which the nitro compound is l-nitropropane.

13. The process of claim 9 in which the nitro compound is nitrobenzene.

14. The process of claim 9 in which the nitro compound is o-nitroanisole.

References Cited in the file of this patent UNITED STATES PATENTS 2,227,833 Hixson et a1. Oct. 12, 1945 

1. A PROCESS FOR THE SEPARATION OFPLUTONIUM FROM AN AQUEOUS SOLUTIN CONTAINING PLUTONIUM IN AN OXIDATION STATE OF AT LEAST +4 WHICH COMPRISES CONTACTING SAID SOLUTION WITH A SOLVENT SELECTED FROM THE GROUP CONSISTING OF NITROMETHANE, NITROETHANE, 1-NITROPROPANE, NITROBENZENE AND O-NITROANISOLE, AND SEPARATING THE RESULTING EXTRACT AND RAFFINATE PHASES. 